The present invention relates to a method for reduction of particulate and gaseous contaminants from exhaust gas. While much of this disclosure will refer to coal fired power plants, the present system and method is not limited to exhaust gas that is the result of the combustion of coal, and further is not limited to flue gas associated with the combustion of other fuels such as natural gas, diesel, waste oil, garbage, and the like, rather the present system and method is applicable to any exhaust gas stream from which it is desirable to remove contaminants, particularly any gas stream application that may currently utilize precipitator, scrubber, and any other, technologies to remove contaminants.
With that out of the way and now referring back to combustion exhaust gas, particulate matter carried in suspension by the effluent or waste gases from furnaces burning fossil fuels is commonly referred to as fly ash. Fly ash is an undesired by-product of coal fired power plants. The fly ash created by power plants can greatly vary depending on the type of coal used as fuel. For instance lignite coal produces Class C fly ash that is high in lime (CaO), which is commonly formed into blocks. In another example, anthracite and bituminous coals produce Class F fly ash which is naturally low in lime (CaO). As a result, power plants, and their pollution control systems, are generally designed specifically for the type of fuel used in order to adequately to reduce particulate and gaseous contaminants from the exhaust gas. In the past, the dumping of untreated power plant exhaust gas resulted in acid rain that damaged buildings and plants, introduced high concentrations of heavy metals such as mercury (Hg) and cadmium (Cd) in the environment, as well as particulate dust that covered buildings and proved hazardous to persons with respiratory problems. Due to the serious damage untreated exhaust gases have on the environment, EPA regulations have been implemented to prevent the dumping of untreated power plant exhaust gases directly into the environment. Unfortunately, power plants create large amounts of fly ash each day that must be properly disposed of. For example, a power plant with large boiler rated at 1,400,000 lb./steam/hr. typically will emit 700,000 cfm of waste gas and 140 tons of ash/day. Some plants that produce Class C fly ash turn a portion of the fly ash into cinder blocks which helps to recoup a portion of the added expense of fly ash disposal. Power plants that produce Class F fly ash, due to low lime content of the ash, a binder such as Portland cement must be added to the fly ash in order to create cinder blocks, as a consequence costing the power plant more money to dispose of the fly ash. Currently, 3 to 4% of the total capital investment goes to high-efficiency ash-collecting and handling equipment.
Due to the need to prevent damage to the environment from pollutants and fly ash emitted from power plants, almost all pulverized coal power plant boilers incorporate high efficiency exhaust gas cleaning equipment. Many factors determine collection efficiency of a power plants fly ash and other pollutants. For instance, mechanical feasibility, the footprint of the allocated land, and profitability all play a role in determining what equipment a power plant can use and the pollutant collection efficiency.
Fly ash collection equipment usage has increased as boilers are designed to use coal with higher ash content and increased output. For example, earlier power plant installations had a 90% collection efficiency requirement, whereas modern power plants have 95 to 98% fly ash and pollutant collection efficiency. Variability in fly ash characteristics complicates fly ash and pollutant collection even with the advancement of power plant furnace design and pollution collection methods.
Coal fired power plants use a plurality of methods and equipment to reduce fly ash and other pollutants from being emitted into the environment. For instance, electrostatic precipitators are commonly used to separate particulate matter from the exhaust gas. Electrostatic precipitators use a direct current high voltage to induce a charge on particles in the exhaust gas. The charge causes the particles to move towards and stick to grounded plates in the electrostatic precipitators. Unfortunately, electrostatic precipitators are only effective in removing particulate matter, such as fly ash, from the exhaust gas stream. Vertical wet scrubbers are also used to clean the exhaust gas stream of particulate matter and other pollutants. In a vertical wet scrubber, exhaust gases flow in an upwards fashion while a mist in sprayed into the exhaust gas stream. As the mist travels down the vertical wet scrubber tower, the mist collects particulate matter in the exhaust gas stream and deposits them in the bottom of the tower. Additionally, various chemical agents may be included in the mist to react with pollutants in the exhaust gas. Some of these pollutants include sulfur dioxide (SO2), mercury (Hg), and other heavy metals. Such systems are illustrated in FIG. 1.